Alesis Power Amp Transistors Dead what else to check? - diyAudio
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Old 13th March 2008, 09:07 PM   #1
777funk is offline 777funk  United States
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Default Alesis Power Amp Transistors Dead what else to check?

I just tore apart an Alesis 500 watt power amp. I found 3 of 4 PNP's on on Channel of the power transistors to be shorted. And 1 of 4 NPN's was also dead. So of 8 power transistors 4 are dead.

What should I change besides those 4 dead transistors? Are there other parts I need to check?

I think some kids turned it on with one of the channels unplugged from a speaker connection???

Anyways, this is a good friend's amp and it'd be nice to see it come back to life. I know it has at least 4 dead Output Transistors but I don't want to just throw $25 worth of parts at it to find that something else is going to kill the new transistors.

Thanks in advance for any help!
Nick
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Old 13th March 2008, 09:20 PM   #2
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First, when repairing an amplifier which's output is blown I always recommend to replace all the power output tansistors (not only the shorted ones!).

Any blown fuse(s)?
Are the drivers/predrivers survived the amp's failure?
Any burned passive part on the PCB?

What happened? Unplugged speakers theoretically couldn't cause an amplifier blowing up!
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Old 13th March 2008, 09:46 PM   #3
777funk is offline 777funk  United States
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I have no idea what happened to this thing. But my friend tells me that the output load was plugged into who knows what while play keyboards??? Maybe??? through this thing. There's a 1/4 or 1/2 watt Emitter resistor that looks burnt. It's metal film. It still measures fairly close to the other emitter resistors.

Also, the transistors are MJL21193's and 94's. Any good source for those?

They're about $5 each at mouser and there's 8 of them. That's $40 worth of just transistors.

Do I really need to replace even the good ones?

Quote:
Originally posted by Andy L. Francis
First, when repairing an amplifier which's output is blown I always recommend to replace all the power output tansistors (not only the shorted ones!).

Any blown fuse(s)?
Are the drivers/predrivers survived the amp's failure?
Any burned passive part on the PCB?

What happened? Unplugged speakers theoretically couldn't cause an amplifier blowing up!
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Old 13th March 2008, 10:08 PM   #4
Jeb-D. is offline Jeb-D.  United States
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Check the driver transistors
Check emitter resistors (with ohm meter)
Check base resistors (on some amps they use resistors here that open without looking burnt, check with multi-meter)
Look for obviously burnet components
Check fuse
Check bias compensation transistor ( I know it doesn't make sense as to why, but I've seen them go many times)
If you really want to be safe check all the transistors(the diode checker on a multi meter works good for this, check the PN junctions)

It can cause the output transistors to blow again if one of the other parts is blown. Although it is also common for only the output transistors to burn out.
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Old 14th March 2008, 01:53 AM   #5
Leolabs is offline Leolabs  Malaysia
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Also check for bad caps esp those near the output stages and feedback loops.
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Old 14th March 2008, 10:45 AM   #6
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Quote:
Originally posted by Leolabs
Also check for bad caps esp those near the output stages and feedback loops.
It's a repair-project of an amplifier that's OP stage blown. It's not a restoration of an old amplifier, or it's not an SMPS/TV where a bad cap could cause the device to blow up.
Do you think that this Alesis amp blown up because of a bad elko?
In my opinion searching for bad caps is unnecessary in this case.
If it would be a television receiver of course I'd start with checking elkos in the PSU.
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Old 14th March 2008, 02:38 PM   #7
Leolabs is offline Leolabs  Malaysia
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Then guess that someone havent seen a blown amp put too much DC on the feedback to ground cap and bias bypass cap.
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Old 14th March 2008, 04:35 PM   #8
wg_ski is offline wg_ski  United States
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Replace ALL outputs, both drivers, the bias stack, any resistor or diode near the OPS that looks funny or doesn't mesure right. Before going any further. You will likely save hours of frustration.
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